There's great documentation on how to install BES for Exchange 2007 ... but I'm thinking most people are migrating from 2003 to 2007. Here's a fantastic KB Article RIM has posted...
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How to migrate the BlackBerry Enterprise Server from Microsoft Exchange Server 2000 or 2003 to Microsoft Exchange Server 2007
Doc ID :
KB14502
Last Modified : 2007-11-07
Document Type : How To
Environment
* BlackBerry® Enterprise Server for Microsoft® Exchange
* Microsoft® Exchange Server 2000
* Microsoft® Exchange Server 2003
* Microsoft® Exchange Server 2007
Procedure
To migrate the BlackBerry Enterprise Server from Microsoft Exchange Server 2000 or 2003 to Microsoft Exchange Server 2007, complete the following:
1. Stop all services for the BlackBerry Enterprise Server.
2. Move the BlackBerry Enterprise Server service account (default name is BESAdmin).
3. After migrating the Microsoft Exchange Server, verify the Microsoft Exchange Server 2007 permissions for the BlackBerry Enterprise Server.
4. Set the required permissions for the BlackBerry Enterprise Server in Microsoft Exchange 2007. For instructions, see KB12483.
5. If Microsoft Exchange System Manager 2003 is installed on the BlackBerry Enterprise Server, remove it.
6. Perform a search for the files: MAPI32.dll and cdo.dll.
7. Rename any cdo.dll and MAPI32.dll files from the system32 and program files\exchsrvr\bin folder to a .bak file extension.
8. Download and install the latest Messaging Application Programming Interface (MAPI) and CDO clients. For more information, see KB12697.
9. Install a MAPI subsystem by downloading and installing the Microsoft Exchange Server MAPI Client and Collaboration Data Objects 1.2.1. For instructions, see the Microsoft Support web site.
10. Restart the BlackBerry Enterprise Server or run the handheldcleanup utility. For instructions on running the handheldcleanup utility, see KB00140.
11. Resolve the MAPI profile for the BESAdmin mailbox on the Microsoft Exchange Server 2007 by going to Start > Programs > BlackBerry Enterprise Server > Edit the MAPI.
Important: Restarting the BlackBerry Enterprise Server will delay email message delivery to BlackBerry smartphones. For more information, see KB04789.